The present invention relates to a nozzle for blowing out an electric arc, this nozzle being designed to be incorporated either in a medium-voltage circuit breaker, the voltage of which typically lying in the range 7.2 kV to 52 kV, or in a high-voltage circuit breaker, the voltage of which typically lying in the range 52 kV to 800 kV.
The invention also relates to a high-voltage circuit breaker fitted with such an electric arc-blast nozzle.
An arc-blast circuit breaker comprises at least two arcing contacts that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts are separated from each other and a closed position of the circuit breaker in which the arcing contacts are in contact with each other; an electric arc-blast nozzle; and an arc-control gas flowing in the nozzle in order to interrupt an electric arc that is likely to form during movement of the arcing contacts from the closed position to the open position of the circuit breaker.
A conventional electric arc-blast nozzle comprises the following portions:
a middle portion forming a throat defining internally an axial passage for breaking an electric arc, and two end portions extending on either side of the middle portion and being designed to receive respective arcing contacts that are movable axially relative to each other, between an open position of the circuit breaker in which the arcing contacts are separated from each other and a closed position of the circuit breaker in which the arcing contacts are in contact with each other and in which one of the arcing contacts partially closes the axial passage of the middle portion, an arc-control gas flowing through the axial passage of the middle portion in order to interrupt an electric arc that is likely to form during movement of the arcing contacts from the closed position to the open position of the circuit breaker.
In order to interrupt an electric arc, an arc-blast circuit breaker uses an arc-control gas formed by an insulating dielectric gas. This arc-control gas is delivered from a blast chamber into the axial passage of the middle portion of an above-described electric arc-blast nozzle. Such a nozzle has the function of channeling the electric arc and, by doing so, of increasing the pressure of the arc-control gas in the vicinity of the electric arc, thus promoting arc extinction.
Currently, the most frequently-used arc-control gas for that type of circuit breakers is sulfur hexafluoride SF6 because of the exceptional physical properties of said gas. However, SF6 presents the major drawback of being a very powerful greenhouse gas, with a particularly high global warming potential (GWP).
Among the alternatives to the use of SF6 as an arc-control gas, various gases of global warming potential (GWP) that is lower than that of SF6 are known, such as dry air or also nitrogen.
A particularly advantageous arc-control gas is carbon dioxide CO2 because of its strong electric insulation and arc-extinction capabilities. Furthermore, CO2 is non-toxic, non-flammable, with a very low GWP, and is also easy to procure.
CO2 can be used alone or in the form of a gas mixture, of which it constitutes the main gas referred to as “vector gas”.
But, and contrary to the SF6 which has the property of recombining after decomposition by arc discharge, CO2 cannot recombine and namely produces a significant amount of gaseous toxic carbon monoxide CO and of carbon powder.
In order to remove such a toxic CO gas, several solutions have been proposed.
A first solution consists in providing the arc-blast circuit breaker with a synthetic zeolite as a CO adsorptive agent.
The drawback of that first solution lies in the fact that such a zeolite cannot sufficiently remove CO as the zeolite also adsorbs the insulating CO2 gas.
As a second solution, document EP 2 779 195, referenced [1] hereinafter in the present description, discloses an electric arc-blast nozzle provided with a metal oxide at a portion contacting with a heat stream generated by an arc discharge that is likely to form during movement of the arcing contacts from the closed position to the open position of the arc-blast circuit breaker. The metal oxide disposition method may include a method of forming the contacting portion with a metal oxide, a method of coating the contacting portion with a cover material of a metal oxide or a method of coating the contacting portion with a metal oxide film.
According to Document [1], the contacting portion should reach a temperature of at least 200° C. for obtaining the recombination of CO gas into CO2 gas. Indeed, in such temperature conditions, the metal oxide located in the contacting portion acts as an oxidizer. An example of the corresponding reaction, with MnO2 as metal oxide, follows:2C+MnO2→2C+Mn
For obtaining such a reaction, the contacting portion with the metal oxide is placed in an area that is very close to the electric arc in order to reach the required temperature of at least 200° C. However, this contacting portion, which is in contact with the gas stream and subjected to a temperature of at least 200° C., is a very confined area. The corresponding metal oxide content is consequently relatively limited and, in most cases, not sufficient to convert CO gas into CO2 gas.
In addition, circuit breakers are today designed to interrupt an electric arc in a few milliseconds (ms), typically between 5 ms and 25 ms. But, heat transfer by means of conduction cannot occur within such a time range and only heat transfer by convection and by radiation contributes to a temperature rise of the contacting portion. The corresponding heat flow thus remains superficial and does not allow a good conversion of CO gas into CO2 gas.
Additionally, it must be noted that Document [1] is totally silent on the carbon powder deposit, such a deposit impairing the dielectric strength of the electric arc-blast nozzle.
The invention therefore aims to propose a novel electric arc-blast nozzle that enables the drawbacks of prior art electric arc-blast nozzles to be mitigated.
In particular, this new nozzle must be suitable for fitting to a circuit-breaker operating with CO2 alone or with a gas mixture including CO2 as vector gas, such a circuit-breaker being provided with a conversion of CO gas into insulating CO2 gas that is higher than the one of the electric arc-blast nozzle disclosed by Document [1].
The new nozzle must also be suitable for fitting to such a circuit breaker without any increase in its bulk and without any noticeable increase in costs, namely in terms of manufacturing process, as it is the case for the electric arc-blast nozzle disclosed by Document [1].